Why Use Clustering with Hyper-V?
In the previous sections, I went into a lot of detail about quorum and how clusters
work. The key point is this: Clusters help keep the workloads available with a minimal
amount of downtime, even in unplanned outages. For Hyper-V servers that are
running many virtual machines, keeping the virtual machines as available as possible
is critical.
When looking at high availability, there are two types of outage: planned and
unplanned. A planned outage is a known and controlled outage—for example, when
you are rebooting a host to apply patches or performing hardware maintenance or
even powering down a complete datacenter. In a planned outage scenario, it is
possible to avoid any downtime to the virtual machines by performing a Live
Migration of the virtual machines on one node to another node. When Live Migration
is used, the virtual machine is always available to clients.
An unplanned outage is not foreseen or planned, such as in the case of a server crash
or hardware failure. In an unplanned outage, there is no opportunity to perform Live
Migration of virtual machines between nodes, which means that there will be a period
of unavailability for the virtual machines. In an unplanned outage scenario, the
cluster will detect that a node has failed and the resources that were running on the
failed node will be redistributed among the remaining nodes in the cluster and then
started. Because the virtual machines were effectively just powered off without a clean
shutdown of the guest OS inside the virtual machines, the guest OS will start in what
is known as a crash-consistent state, which means that when the guest OS starts and
applications in the guest OS start, there may be some consistency and repair actions
required.
In Windows Server 2008 R2, the Live Migration feature for moving virtual machines
with no downtime between servers was available only between nodes in a cluster,
because the storage had to be available to both the source and target node. In
Windows Server 2012, the ability to live-migrate between any two Hyper-V 2012 hosts
was introduced. It’s known as Shared Nothing Live Migration, and it migrates the
storage in addition to the memory and state of the virtual machine.
One traditional feature of clustering was the ability to move storage smoothly between
nodes in a cluster. It was enhanced greatly with Windows Server 2008 R2 to allow
storage to be shared between the nodes in a cluster simultaneously; it’s known as
Cluster Shared Volumes (CSV). With CSV, an NTFS volume can be accessed by all of
the nodes at the same time, allowing virtual machines to be stored on a single NTFS-
formatted LUN and run on different nodes in the cluster. The sharing of storage is a
huge feature of clusters, and it makes the migration of virtual machines between
nodes a much more efficient process because only the memory and state of the virtual
machine needs to be migrated and not the actual storage. Of course, in Windows
Server 2012, nodes not in a cluster can share storage by accessing a common SMB 3